This invention relates to an exposure apparatus used in a photolithography process for manufacturing, for example, semiconductor devices, liquid crystal display devices, image pick-up devices, thin-film magnetic heads, and the like.
In manufacturing a semiconductor device or the like using a photolithographic technique, a step-and-repeat type exposure apparatus has been conventionally used, in which a pattern of a photomask or a reticle (referred to as a mask) is projected and exposed through a projection optical system onto each shot area of a photosensitive substrate. Examples of the photosensitive substrate include a semiconductor wafer or a glass plate on which a photosensitizer (e.g., photoresist) is applied.
A photosensitive substrate is loaded on a substrate stage and moved within a two-dimensional plane, which is perpendicular to the optical axis (Z direction) of the projection optical system. A pair of moving mirrors are fixed onto the substrate stage. A pair of laser interferometers measure a distance from one of the moving mirrors, respectively, thereby detecting the coordinates of the substrate stage within the XY plane. A substrate stage control system drives the substrate stage by a predetermined amount in a stepwise manner within the coordinate system defined by the laser interferometers, so that each shot area of the photosensitive substrate is brought into alignment with the exposure field of the projection optical system.
A scanning type exposure apparatus has been developed, which scans the mask and the photosensitive substrate in a synchronized manner with respect to the projection optical system. This type of exposure apparatus allows a pattern to be exposed onto a shot area that is broader than the effective exposure field of the projection optical system. The scanning type exposure apparatus can be of a collective exposure type or a step-and-scan type. In a collective type exposure apparatus, a pattern of a mask is projected and exposed onto the entire area of a photosensitive substrate at a magnification ratio of one. In a step-and-scan type exposure apparatus, a mask pattern is exposed onto a single shot area of a photosensitive substrate at a certain reduction ratio, and when exposure of one shot area has been completed, the next shot area is brought into the exposure field in the stepwise manner.
In any type of exposure apparatus, a mask and a photosensitive substrate must be precisely aligned to superpose the mask pattern onto a pattern that has already been accurately formed on the photosensitive substrate. Generally, alignment sensors are provided in an exposure apparatus to detect a mask alignment mark that is formed on a mask and a substrate alignment mark that is formed on a photosensitive substrate. Based on the detected position of the alignment marks, the photosensitive substrate is aligned with the mask.
Alignment sensors used in the exposure apparatus include a TTL (through-the-lens) sensor system for detecting the position of the photosensitive substrate through the projection optical system, a TTM (through-the-mask) sensor system for detecting a positional relation between the mask and the photosensitive substrate through the projection optical system and the mask, and an off-axis sensor system for directly measuring the position of the photosensitive substrate without using the projection optical system. A reference mark is provided on the substrate stage for calibrating the alignment sensors and detecting a distance between the center of the projected image of the off-axis alignment system and the center of the projected image of the projection optical system, which is a so-called base-line amount.
In general, a projection optical system used in an exposure apparatus has a large numerical aperture (NA) and a shallow focal depth. In order to transfer a minute pattern onto a photosensitive substrate with high resolution, a mechanism is required for bringing the surface of the photosensitive substrate into an image-forming plane of the projection optical system. To this end, an oblique-incident type multipoint autofocus (AF) system is provided to detect the focal point (i.e., the position along the optical axis) of a shot area on the photosensitive substrate and an inclination of the surface of the shot area. With the oblique-incident type autofocus system, a plurality of measurement points are set within a shot area of the photosensitive substrate, and a plurality of slit images are obliquely projected to the measurement points. The slit images reflected by the measurement points are formed on a photodetector. A focal point and an inclination of the shot area are determined from the image-forming positions of the slit images on the photodetector. Based on the detection result of the multipoint AF system, autoleveling control for making the surface of a shot area parallel to the focal plane of the projection optical system, and autofocus control for bringing the focal position on the surface of the photosensitive substrate into the focal position of the projection optical system are performed. In this manner, each shot area is brought into an acceptable range of the focal plane of the projection optical system.
As a mask pattern is repeatedly exposed onto respective shot areas of a photosensitive substrate, the temperature of the photosensitive substrate rises because of the exposure energy of the illumination light. Moreover, when the photoresist layer formed on the photosensitive substrate is exposed, a photochemical reaction is caused within the photoresist. If the photochemical reaction is an exothermic reaction, the temperature of the photosensitive substrate further increases. Since the photosensitive substrate thermally contacts the substrate stage, heat generated in the photosensitive substrate is transferred to the substrate stage through conduction so that the photosensitive substrate and the substrate stage are in thermal equilibrium.
A portion of the heat generated in the photosensitive substrate and transferred to the substrate stage is released in the air surrounding the photosensitive substrate and the substrate stage. However, most of the heat is accumulated on the substrate stage through the repeated pattern exposure process. As a result, the temperature of the substrate stage rises. The temperature rise in the substrate stage causes two major problems.
First, alignment between the mask and the photosensitive substrate is adversely affected. As has been mentioned above, various types of alignment sensors are used in an exposure apparatus, which are calibrated using a reference mark provided on the substrate stage. The reference mark is used by the off-axis alignment system to control the base-line amount. The reference mark is made of, for example, a quartz glass, on which a pattern is drawn by chromium and is fixed to the top surface of the substrate stage. If the temperature of the substrate stage changes, the reference mark slightly rotates.
Moving mirrors are also fixed to the substrate stage to measure the X and Y coordinates of the substrate stage. When the temperature of the substrate stage rises, the position and the fixing angle of the moving mirrors change due to thermal deformation of the supporting member of the moving mirrors. If the position or fixing angle of the moving mirror changes, the reference mark rotates relative to the moving mirror, which affects the base-line measurement. Deformation of the supporting member of the moving mirror causes errors in the orthogonality of the coordinate system, as well as an offset amount.
Second, the autofocus function is adversely affected. An exposure apparatus is generally positioned in a chamber in which the atmospheric temperature is maintained constant by a temperature adjuster. If the temperature of the substrate stage rises, the air surrounding the substrate stage wavers due to a temperature difference between the atmosphere and the substrate stage. An oblique incident AF detection system emits a detection beam obliquely with respect to the photosensitive substrate loaded on the substrate stage, and detects a beam reflected by the surface of the photosensitive substrate. If the air wavers around the substrate stage, the detection accuracy of the AF system drops due to the fluctuation of the air in the optical path of the detection beam. As a result, the autofocusing function of the apparatus deteriorates.
To cool the substrate stage, liquid cooling or air cooling may be considered. With liquid cooling, cooling tubes are attached to the substrate stage, through which a coolant is supplied. The substrate stage, however, generally includes various mechanisms, such as X and Y stages for moving the photosensitive substrate within the XY plane, a Z stage for moving the photosensitive substrate in the Z direction to perform autofocus control, a tilting mechanism for tilting the substrate-loading plane to level the exposed surface of the photosensitive substrate, and a loading/unloading mechanism for transferring the photosensitive substrate between the substrate stage and a substrate transporting mechanism. If cooling tubes are attached to the substrate stage, the structure of the substrate stage becomes further complicated. Moreover, whenever the substrate stage moves, the cooling tubes are trailed between the substrate stage and the pump for supplying a coolant, which imposes a large amount of load on the stage driving unit. On the other hand, the alternative air cooling method is inferior in cooling efficiency.